The present invention relates generally to microcalorimeters and more specifically to features that improve the performance of microcalorimeters, especially an automated isothermal titration micro calorimetry system (ITC system).
Microcalorimeters are broadly utilized in fields of biochemistry, pharmacology, cell biology, and others. Calorimetry provides a direct method for measuring changes in thermodynamic properties of biological macromolecules. Microcalorimeters are typically two cell instruments in which properties of a dilute solution of test substance in an aqueous buffer in a sample cell are continuously compared to an equal quantity of aqueous buffer in a reference cell. Measured differences between the properties of the two cells, such as temperature or heat flow, are attributed to the presence of the test substance in the sample cell.
One type of microcalorimeter is an isothermal titration calorimeter. The isothermal titration calorimeter (ITC) is a differential device, but operates at a fixed temperature and pressure while the liquid in the sample cell is continuously stirred. The most popular application for titration calorimetry is in the characterization of the thermodynamics of molecular interactions. In this application, a dilute solution of a test substance (e.g., a protein) is placed in the sample cell and, at various times, small volumes of a second dilute solution containing a ligand, which binds to the test substance, are injected into the sample cell. The instrument measures the heat, which is evolved or absorbed as a result of the binding of the newly introduced ligand to the test substance. From results of multiple-injection experiments, properties, such as, the Gibbs energy, the association constant, the enthalpy and entropy changes, and the stoichiometry of binding, may be determined for a particular pairing between the test substance and the ligand.
While currently utilized ITCs provide reliable binding data results, their widespread utilization in the early stages of drug development have been limited by several factors: the relatively high amounts of protein required to perform a binding determination (e.g., about 0.1 milligram (mg) to about 1.0 mg of a protein), the limited throughput due to the time required to perform the measurement and the complexity of using conventional ITCs.
Today, gathering binding data utilizing prior art ITCs require extensive preparation and skill by the practitioner. For example, using prior art ITCs, the reference and sample cells are first filled respectively with the reference substance and sample substance via a corresponding cell stem. Next, a titration pipette of the ITC is filled with a titrant, which is a delicate operation as it is very important that the syringe in the pipette is accurately filled and that there is no air trapped therein. Then a needle of the titration pipette is manually placed in the sample cell via the cell stem, and the ITC experiments can be initiated. The ITC measurement procedure is controlled by a control unit in the form of a computer or the like running a program for performing the experiments. Consistent with the program used for the experiment, a stirring motor rotates the syringe, needle, and paddle at an assigned speed enabling proper mixing of the reagents. Consistent with the program used for the experiment (e.g., when a certain temperature and/or equilibrium are reached), a plunger in the syringe is activated to inject the titrant into the sample solution. The injection can be done discretely (step-by-step) or continuously, depending on the program settings. The calorimeter continuously measures and records the heat release/absorption versus time associated with the interaction of reagents. The analysis of the results is done according to the established algorithm.
As would be appreciated by a reading of the above-described prior art procedure, utilizing prior art ITCs, the quality of binding measurements performed with these prior art ITCs depends heavily of the operator's skills and experience, and involves a considerable amount of preparation time.
For some time there has been at least one automated ITC system on the market, MicroCal AutoITC, which is based on a commercially available micro calorimeter and a linear robot system and a fluidics system arranged to perform automatic sample handling.